This invention relates generally to the detection of cracks in the rotor of a turbine-generator set and in particular to a method for the detection of incipient rotor cracks while the turbine-generator is on-line under normal load.
The rotor of a turbine-generator (for example, the rotor of a large steam driven turbine-generator set of the type used in the commercial generation of electrical power) is frequently subjected to severe mechanical and thermal stresses due to any number of fairly routine operating conditions. Among these are such things as changes in load and operating temperature. While the rotors of these machines have an excellent record of safety and reliability, and are designed to withstand these stressful conditions, cracks have developed under some circumstances in some rotors following years of service. The development and growth of a crack is by no means predictable, however, and in some extreme cases rotors have actually burst in a brittle fracture mode.
Obviously, a cracked rotor must be replaced or repaired to protect equipment and personnel and to insure continued efficient operation. If a crack is discovered soon enough, while it is small, the rotor may be economically repaired and returned to service in a relatively short period of time. On the other hand, if the crack has grown to the point that the rotor must be replaced, there is not only the high cost of replacement but there is a potentially greater cost resulting from the loss of power generating capacity. Thus, there has been a continual search in the art for improved methods and apparatus by which an incipient crack can readily be detected in the rotor of a turbine-generator so that corrective action can be quickly taken.
A number of techniques are known in the art and used to detect the presence and growth of a rotor crack. These include (1) surface inspection methods such as magnetic particle testing, eddy current testing, and dye penetrant techniques; and (2) volumetric methods such as ultrasonic testing (audiography). Unfortunately, none of these techniques is suitable for inspection while the machine is on-line running under load. With only these techniques available, operating personnel have tried to insure themselves against a major loss by scheduling periodic outages during which the turbine-generator is inspected and tested by such techniques. While these periodic inspections provide an assessment of the condition of a rotor as it is at the time the investigation is performed, there is always the risk that a crack might initiate and grow between inspections.
Another known technique, based on vibration signature analysis, has also been used for rotor crack detection and has offered some relief from the need to bring the machine to a complete stop. Until recently this technique has been useful only by taking the machine off line and then decelerating it to nearly zero speed to determine if a crack is present. In a copending application, Ser. No. 236,007 which is of common assignee with the present invention, an on-line signal analysis method of crack detection is disclosed which eliminates the need to decelerate the machine. Indeed, with the method of the aforementioned application, the integrity of the rotor may be determined while the turbine-generator is operating at normal speed and supplying power to a load. However, even with this recently developed on-line signature analysis method, there is a need to transitorily perturb the rotor so that any crack present will manifest itself by producing a new and different vibration response mode. Thus, while the rotor perturbation and subsequent signature analysis of the vibration signal allows detection of incipient cracks on-line, it is generally desirable to avoid such perturbations since there is the potential (if the method is misused) for imposing harmful mechanical or thermal stress on the rotor. Furthermore, since the method is based on a transient response, it does not lend itself to continuous, unattended monitoring.
Accordingly, it is among the objectives of the present invention to provide a rotor crack detection method which is useful for detecting incipient rotor cracks while the turbine-generator is on-line operating under normal load, which is more sensitive than previous methods, and which avoids the need to impose transient, physical perturbations on the rotor.
Another objective of the invention is to provide a method for on-line continuous monitoring of a turbine-generator for early detection of cracks in the rotor.
Still further objectives and advantages will be apparent from the ensuing description of the invention, its principles, and its operation.